1. Field of the Invention
The present invention relates to an image processing method capable of improving color reproducibility, an image processing apparatus therefor, a storage medium therefor, and a program therefor.
2. Description of the Related Art
Some recent copying machines have been used as MFPs (Multi Function Printers), together with printers, by being connected to a network.
In recent years, color matching of printed images between devices connected to a network, or color matching of the color of images displayed on a display device, such as a CRT, and the color of printed images is often performed. Various color management methods for that purpose are known. For example, in color management using ICC (International Color Consortium) profiles, calibration (also called color matching, or characterization) is performed by creating a device ICC profile unique to a printer, a copying machine, or the like. For example, by performing color conversion by personal computer (PC) in order to generate print data and by outputting this print data to a device corresponding to the profile, color matching of the color of the printed image and the color of an image displayed on a display device, etc., becomes possible. Since software and colorimeters for creating profiles are commercially available for general users, an environment in which the color output by an image forming apparatus such as a printer is made to match the target color is becoming available. As another calibration method, calibration in which the contents of a gamma LUT regarding gradation characteristics are changed to obtain desired gradation characteristics without using color conversion based on a multi-dimensional LUT of an ICC profile has been performed.
As described above, color management is an effective method because the difference of output colors between a plurality of devices of the same type and different types can be reduced; however, the range of application is not limited to the foregoing and includes the case where, for example, a printer is used for color calibration by causing the printed color to match the color to be printed by an offset printer. If the ICC profiles for one printing device and a printer are provided, color management such as that shown in, for example, FIG. 15 becomes possible with PC application software.
As shown in FIG. 15, the contents of an ICC profile for printing and an ICC profile for a printer are each calibrated in such a manner as to correspond to, for example, the CIE L*a*b* color space (CIE is the abbreviation of Commission Internationale d'Eclairage), which is a color space that is not dependent on a printer on the basis of the color measurement of patches by using a calorimeter. As a result, it is possible to cause the color to be printed by a printing device and the color printed by a printer to match each other. Then, it becomes possible for the color management module (CMM) to generate print data by performing color conversion using these profiles.
As described above, since a color management environment for calorimeters, application software, profile creation software, etc., is available, image forming apparatuses of an electrophotographic method are increasingly used for color calibration of printing devices in the manner described above in the design industry.
On the other hand, color adjustment for copying machine engines will now be described. In order to make density and gradation reproducibility of copied images and printed images uniform, it is necessary to make corrections by adjusting the following variations:
(1) Short-term variations resulting from variations in the device environment; and
(2) Long-term variations resulting from changes over time of a photosensitive member and a developing agent.
As a specific method, as described in Japanese Unexamined Patent Applications Publication Nos. 10-28229 and 11-75067, first, a test print is formed, a correction coefficient of a contrast potential for forming an image is optimized on the basis of the obtained density information, and a grid voltage and a development bias voltage are set so as to obtain a desired maximum contrast. After this setting, a single-color gradation patch is output, the density is calculated by a reader section, and a one-dimensional LUT (gradation correction table) is generated to form desired targets (density linearity, lightness linearity, etc.).
However, even when the gradation correction of the above-mentioned single-color one-dimensional (C, M, Y) is performed, there are cases in which gradation characteristics of a secondary color (R, G, B) vary due to environmental conditions, variations in the transfer efficiency of a paper type, the degree of deterioration of a fixing roller, or the like. FIG. 6 shows CIE (Commission Internationalde l'Eclairage) chromaticity coordinates (a*b*space) showing gradation characteristics of a secondary color and a primary color, which are output after single-color gradation correction. As shown in FIG. 6, even if a secondary color equal-amount signal (for example, a signal having Y and M levels of 30% and 30%) is input, output is performed such that the hue angle of the image formed as a result of the above varies.
As described above, there are many matters to be considered, such as the red gradation which is often used in DTP (Desk Top Publishing) not being output correctly, and the smoothness of flesh-color portions and the color matching accuracy being low due to variations of the hue angle of the secondary color image formed.
Furthermore, chroma spacing of single color (primary color) is maintained constant by single-color gradation correction, and single-color gradation characteristics are preferable; however, hue variations of the secondary color occur as in the above-described problem. When the visual-angle differential limen is taken into consideration, greater importance should be placed on hue than chroma, since hue-angle variations of the secondary color are more conspicuous than chroma variations of a single color. Therefore, there have been demands for gradation correction in which greater importance is placed on the hue angle.